Semi-automatic firearms, such as rifles and shotguns, are designed to fire a round of ammunition, such as a cartridge or shotshell, in response to each squeeze of the trigger of the firearm, and thereafter automatically load the next shell or cartridge from the firearm magazine into the chamber of the firearm. During firing, the primer of the round of ammunition ignites the propellant (powder) inside the round, producing an expanding column of high pressure gases within the chamber and barrel of the firearm. The force of this expanding gas propels the bullet/shot of the cartridge or shell down the barrel.
In semi-automatic and automatic rifles and shotguns that rely on such gases from firing to drive operation of the firearm, gas from a fired cartridge is directed to the bolt carrier or to a piston assembly for driving the bolt carrier to cycle the action of the firearm. For example, upon firing a cartridge in a firearm having a direct gas impingement system, high-temperature, high-pressure gas follows the exiting projectile down the barrel. A portion of the gas from the fired cartridge travels into a port and along a gas tube, rearwardly to a gas key that is coupled to the bolt carrier and includes an internal port to allow the high-pressure gas to flow against the bolt carrier. As the gas expands, the pressure from the gas drives the bolt carrier and bolt. The bolt carrier and bolt thus translate rearwardly against the return spring located in the buttstock, extracting the empty cartridge. Thereafter, forward movement of the bolt and bolt carrier by the return spring loads a next cartridge from the ammunition magazine and returns the bolt. The bolt returns to a locked position for firing.